added gtk theming

modules/system/gtk/default.nix

@@ -0,0 +1,82 @@
+{
+  self,
+  inputs,
+  ...
+}: {
+  perSystem = {
+    pkgs,
+    system,
+    ...
+  }: {
+    packages.catppuccinGtkMochaMauve = pkgs.catppuccin-gtk.override {
+      variant = "mocha";
+      accents = ["mauve"];
+      size = "standard";
+      tweaks = ["rimless"];
+    };
+  };
+
+  flake.nixosModules.gtkTheme = {
+    config,
+    pkgs,
+    lib,
+    ...
+  }: let
+    catppuccinGtk = pkgs.catppuccin-gtk.override {
+      variant = "mocha";
+      accents = ["mauve"];
+      size = "standard";
+      tweaks = ["rimless"];
+    };
+
+    gtkThemeName = "Catppuccin-Mocha-Standard-Mauve-Dark";
+
+    gtkSettingsCommon = ''
+      gtk-application-prefer-dark-theme = true
+      gtk-theme-name = ${gtkThemeName}
+      gtk-icon-theme-name = Catppuccin-Mocha-Mauve
+      gtk-font-name = FiraMono Nerd Font 11
+      gtk-cursor-theme-name = Bibata-Modern-Ice
+      gtk-cursor-theme-size = 20
+    '';
+  in {
+    fonts.packages = [pkgs.nerd-fonts.fira-mono];
+
+    environment.systemPackages = with pkgs; [
+      catppuccinGtk
+      bibata-cursors
+      catppuccin-icons
+    ];
+
+    environment.etc = {
+      "gtk-3.0/settings.ini".source = pkgs.writeText "gtk3-settings.ini" ''
+        [Settings]
+        ${gtkSettingsCommon}
+      '';
+
+      "gtk-4.0/settings.ini".source = pkgs.writeText "gtk4-settings.ini" ''
+        [Settings]
+        ${gtkSettingsCommon}
+      '';
+    };
+
+    environment.variables = {
+      XCURSOR_THEME = "Bibata-Modern-Ice";
+      XCURSOR_SIZE = "20";
+    };
+
+    programs.dconf.profiles.user.databases = [
+      {
+        lockAll = false;
+        settings = {
+          "org/gnome/desktop/interface" = {
+            gtk-theme = gtkThemeName;
+            icon-theme = "Catppuccin-Mocha-Mauve";
+            cursor-theme = "Bibata-Modern-Ice";
+            font-name = "FiraMono Nerd Font 11";
+          };
+        };
+      }
+    ];
+  };
+}